1. Field of the Invention
The present invention relates to a method for magnetic recording using a microwave assisted magnetic head to write data signals on magnetic recording media having a large coercive force to stabilize the magnetization.
2. Description of the Related Art
In association with an advance of high density recording, bit cells for recording digital information on magnetic recording media are miniaturized. As a result, since signals detected by a reproducing element of a magnetic head fluctuate due to so-called thermal fluctuation, a signal-noise ratio (S/N) may be deteriorated or the signal may be lost in the worst case.
Therefore, in recent years, miniaturizing magnetic nanoparticles that configure a recording layer while at the same time raising the magnetic anisotropy energy Ku that pins the magnetic direction of magnetic nanoparticles has been effective at solving the above problem in a magnetic recording medium that uses a practical perpendicular recording system. A thermal stability index S that corresponds to the thermal fluctuation is expressed by the following formula, and normally the thermal stability index S value of at least 50 is said to be required.S=Ku·V/kB·T 
In the above equation, Ku is the “magnetic anisotropy energy,” “V is the “volume of magnetic nanoparticles configuring the recording layer,” kB is the “Boltzmann Constant,” and T is the “absolute temperature.”
However, according to the so-called Stoner-Wohlfarth model, because the magnetic field (magnetization reversal field) Hsw required for recording information is proportional to Ku, raising the Ku causes an increase in Hsw.
To form a reversal of magnetization of the recording layer corresponding to a desired data sequence, it is necessary to apply a recording magnetic field having an intensity that exceeds Hsw that changes steeply. For magnetic disk devices (or hard disk drive, HDD), which are utilized in practice in recent years because of the perpendicular recording system, a recording element with a so-called single magnetic pole is utilized. A recording magnetic field is applied, which is perpendicular to a recording layer from a surface of an air bearing surface (ABS).
This intensity of a perpendicular recording magnetic field is proportional to a saturation magnetic flux density Bs of a soft magnetic material forming the single magnetic pole. Therefore, materials having a saturation magnetic flux density Bs as high as possible are developed and utilized in practice. However, according to the so-called Slater-Pauling curve, Bs=2.4 T (tesla) is a limit of the saturation magnetic flux density Bs for practical use, and currently it is approaching the limit for practical use. A thickness and/or a width of a current single magnetic pole is approximately 100-200 nm. In order to increase a recording density, further reduction of the thickness and/or width is required, and the perpendicular magnetic field generated with such a minute magnetic pole tends to be reduced.
For these reasons, it can be said that the recording ability of the ordinary data writing element is approaching the limit, and that difficulties are faced to achieve the high density recording.
Therefore, a so-called thermal assisted magnetic recording (TAMR) has been proposed. With the TAMR, the recording layer is irradiated with laser light etc., the temperature of the recording layer is increased, and signals are recorded in a situation where the coercive force of the recording layer is lowered.
However, there are the following problems even for the TAMR. (1) A magnetic head providing a magnetic element and an optical element is required so that the configuration thereof is extremely complex and expensive. (2) It is required to develop a recording layer which has a coercive force with a highly sensitive temperature characteristic. (3) Due to a thermal demagnetization during a recording process, adjacent track erasures may occur and/or a recording condition becomes unstable.
In the meantime, research on spin transfer during electron conductivity has been progressing, targeting increased sensitivity of GMR heads and TMR heads as reading elements. Research has been initiated to apply this to magnetization reversal of a recording layer of a magnetic recording medium and to reduce the perpendicular magnetic field required for the magnetization reversal.
Here, a high frequency alternate current magnetic field in an in-plane direction of the magnetic recording medium is applied simultaneously with a perpendicular magnetic field for recording. The frequency of the alternate current magnetic field applied in the in-plane direction is an ultra high frequency (a few up to 40 GHz) of a microwave band that corresponds to a ferromagnetic resonance (also referred to as FMR hereinafter) frequency of magnetic nanoparticles that configure the recording layer of the magnetic recording medium.
Further, analysis results are reported that a reduction in the magnetization reversal field Hsw of the recording layer of about 60% is possible by the simultaneous application of the alternate current magnetic field in the in-plane direction. With the practical use of this method, a configuration that uses a complicated TAMR is not necessary, and it becomes possible to raise the Ku of the recording layer of the magnetic recording medium so that a considerable improvement in recording density is expected.
The phenomenon to decrease the magnetization reversal field appears as the result of precession movement of the spin of the magnetic nanoparticles being excited by the application of an alternate current magnetic field of a frequency close to the spin FMR frequency of the magnetic nanoparticles that configure the recording layer.
However, the spin FMR frequency changes sequentially according to the angle from the easy magnetization axis of the spin. Therefore, by merely applying a single frequency sine wave, the effect of exciting the precession movement is demonstrated only when the spin is at a specific angle in the process of achieving magnetization reversal. In addition, the effect of exciting the precession movement is not demonstrated at other angles of spin because the frequency of the alternate magnetic field does not match the FMR frequency.
Ideally, sequentially changing the assisting microwave frequency by the angle of spin within the precession movement when tracking would be the best method. However, because one cycle of precession movement is a short cycle of 1 ns or less, achieving synchronization and tracking the frequency change of the microwave within this cycle is unrealistic.
To alleviate this type of deficiencies, a method has also been proposed to apply a frequency modulated wave (hereinafter also referred to as an FM wave) to a magnetic recording medium (Japanese Laid-Open Patent Application No. 2010-3339, Tohoku University). However, because single frequency modulation is used, the energy in the spectrum of the FM wave is not uniform and contains gaps. The assistance effect is extremely small when the FMR frequency of magnetic nanoparticles that configure the recording layer enters the gaps in the modulation frequency in this so-called comb-shape state.
In addition, use of an FM wave of the single frequency signal is not desirable as the spectrum intensity weakens when the frequency separates from the center frequency with the FM wave of the single frequency signal, and as the energy of the center frequency may become zero at a certain modulation index.
Furthermore, even if a microwave of an FM wave having a uniform frequency is applied to the magnetic recording medium, the magnetization reversal time is long (approximately about 2 ns), making high speed recording difficult. In addition, when the FM wave is applied, there is a problem that the convergence time for magnetization reversal is lengthened compared to when a microwave having a uniform frequency is applied.